1. Field of the Invention
The present invention relates to an exposure controller, an exposure control method and a camera using the same for automatically controlling the brightness of an image pickup plane, and more particularly to an exposure controller, an exposure control method and a camera equipped with a backlight correcting function.
2. Description of the Related Art
In a camera employing a solid-state image sensor with a CCD or the like as an image pickup device, there is provided an automatic exposure (AE) controller having a backlight correcting function to correct an undesired phenomenon that, when a person at a window for example is shot indoors, the person""s face is rendered extremely dark due to the outdoor brightness. Relative to techniques for such correction of backlight, there is known a method of dividing an image pickup plane into a plurality of frames to detect the amounts of light, then weighting the amount of light in each frame, and taking the weighted average. Another method applying the fuzzy theory is also known.
Out of such backlight correction methods mentioned in the related art, the method of taking the weighted average is disadvantageous in the point that a proper exposure fails to be attained depending on the position of each frame for detecting the amount of light and the position of an object scene. Meanwhile in another method applying the fuzzy theory, there exists a problem that, although a burden of software can be diminished, some difficulties are unavoidable in down-sizing a software processor beyond a certain degree.
It is therefore an object of the present invention to provide improvements in an exposure controller, an exposure control method and a camera adapted for down-sizing a software processor while eliminating the necessity of any complicated process such as functional operation.
According to one aspect of the present invention, there is provided an exposure controller which comprises a detection means for separating an image pickup plane into an object scene portion and a background portion, and detecting the brightness of the object scene portion; and a control means for executing an exposure control action on the basis of the output of the detection means.
In the exposure controller of the above structure, the detection means separates an image pickup plane into an object scene portion and a background portion, and then detects the brightness of the object scene portion to thereby detect the backlight condition of the object scene portion. And the control means executes its exposure control action in accordance with the brightness degree of the object scene portion. Thus, due to such exposure control conforming with the brightness of the object scene portion alone, the object scene being shot in a backlight state is not rendered dark despite the backlight.
According to another aspect of the present invention, there is provided an exposure control method which comprises a step of detecting the average luminance of the entirety of an image pickup plane and then setting first and second reference levels on the basis of such average luminance; a step of separating the image pickup plane into an object scene portion and a background portion on the basis of the first and second reference levels and then detecting the average luminance of the object scene portion; a step of calculating the difference between the average luminance of the entire plane and that of the object scene portion; and a step of executing an exposure control action on the basis of such luminance difference.
In this exposure control method, the average luminance of the entire plane is detected by integrating the light amount, and subsequently first and second reference levels are set through multiplication of the average luminance by predetermined ratios. The image pickup plane is separated into an object scene portion and a background portion on the basis of such first and second reference levels, and the average luminance of the object scene portion is detected. Thereafter the difference between the average luminance of the entire plane and that of the object scene portion is calculated. Since this luminance difference represents the difference between the brightness of the entire plane and that of the object scene portion, an exposure control action is executed on the basis of this luminance difference. In this manner, there is achieved an advantage that, due to such exposure control conforming with the brightness of the object scene portion alone, the object scene being shot in a backlight state is not rendered dark despite the backlight.
According to a further aspect of the present invention, there is provided a camera which comprises an image sensor for converting incident light into an electric signal and outputting the same; an optical system for guiding the incident light from an object scene onto an image pickup plane of the image sensor; a signal processing means for processing the output signal of the image sensor; a detection means for separating the image pickup plane into an object scene portion and a background portion on the basis of the signal supplied from the signal processing means, and then detecting the brightness of the object scene portion; and a control means for executing the exposure control with respect to the image sensor on the basis of the output of the detection means.
In the camera of the above structure, incident light from an object scene is guided by the optical system onto the image pickup plane of the image sensor, and then is converted into an electric signal by the image sensor. The output signal of the image sensor is processed in the signal processing means and then is outputted therefrom as a video signal. Meanwhile the detection means separates the image pickup plane into an object scene portion and a background portion on the basis of the signal obtained from the signal processing means, and then detects the brightness of the object scene portion to thereby detect the backlight condition of the object scene portion. And subsequently the control means executes its exposure control action in accordance with the brightness degree of the object scene portion. Thus, due to such control of the exposure conforming with the brightness of the object scene portion alone, the object scene being shot in a backlight state is not rendered dark despite the backlight.